1. Field of the Invention
The present invention generally relates to a frequency-controlled circuit, and particularly to a frequency-controlled circuit based on a frequency sweep/de-sweep type automatic frequency control (AFC) system, for setting a frequency shift of a received signal, which is developed in a satellite communication device or a main body of a satellite, to a center frequency thereof by sweeping to thereby compensate for the frequency shift.
2. Description of the Related Art
A conventional frequency-controlled circuit based on a general frequency sweep/de-sweep type AFC system is constructed in such a manner that when it is desired to start to pull in or sweep a frequency (compensate for a frequency) firstly after power-up, for example, sweeping is started from a desired or fixed frequency. However, this type of system has a drawback that if a center frequency of a received signal is spaced away from a sweep start frequency or even if it is located close thereto, much time is spent on sweeping the center frequency when the received signal is swept in a direction opposite to a sweeping direction.
In order to avoid the above drawback, a frequency-controlled circuit of such a type as shown in FIG. 9 is known. The frequency-controlled circuit shown in FIG. 9 comprises a voltage-controlled oscillator (oscillator) 1, a mixer 2, a filter 3, an integrator 4, a gain value storage/selection circuit 5 and a demodulator 6.
The oscillator 1 outputs an oscillating signal whose frequency is varied according to a frequency control signal outputted from the gain value storage/selection circuit 5, to the mixer 2. The mixer 2 mixes the oscillating signal and a signal received thereat to thereby obtain a signal of another frequency. Further, the mixer 2 outputs the obtained signal to the demodulator 6 and then outputs it to the integrator 4 through the filter 3 for allowing only the changed frequency signal to pass therethrough. The demodulator 6 demodulates the signal outputted from the mixer 2 and outputs data obtained by the demodulation therefrom.
The integrator 4 integrates the output signal of the mixer 2, which has been transmitted through the filter 3, so as to calculate a gain value (power) thereof. Next, the integrator 4 outputs the gain value to the gain value storage/selection circuit 5. The gain value storage/selection circuit 5 stores the gain value therein and outputs a frequency control signal based on the stored gain value to the oscillator 1. The gain value storage/selection circuit 5 outputs a frequency control signal to the oscillator 1 in such a manner that N gain values of a frequency signal, which are obtained by N-dividing a frequency band of the received signal inputted to the mixer 2, are inputted. Further, the gain value storage/selection circuit 5 detects the maximum one from the N gain values obtained based on the outputted frequency control signal and outputs a frequency control signal to the oscillator 1 so that the AFC (automatic frequency control) is exercised only in the vicinity of the frequency having the maximum gain value.
According to this construction, a power distribution is first roughly measured over a sweeping frequency range of the received signal. Further, the AFC is exercised on the frequency at which the maximum power is detected. It is therefore possible to sweep the center frequency in a short time.
In the conventional aforementioned frequency-controlled circuit shown in FIG. 9, when the received signal is of a PSK (Phase Shift Keying) modulation wave as represented by a frequency spectral map shown in FIG. 10, for example, the neighborhood of a center frequency f.sub.0 (=49 MHz) of the received signal is flat over a wide frequency range (PSK and roll-off filter are often used and the neighborhood of the center frequency becomes flat in this case) and the distribution of power is constant. Therefore, when a frequency band of the received signal is N-divided into frequency slots SR.sub.1 through SR.sub.7 every 500 KHz, for example, and the maximum gain value is determined at each slot of SR.sub.1 through SR.sub.7, a plurality of slots SR.sub.3, SR.sub.4 and SR.sub.5 each having the maximum gain value exist. Therefore, a problem arises that an area at which the center frequency f.sub.0 exists, cannot be accurately detected.